Changing the temperature surrounding a crystal unit produces thermal gradients
when, for example, heat flows to or from the active area of the resonator plate
through the mounting clips. The static f vs. T characteristic is modified by
the thermal-transient effect resulting from the thermal-gradient-induced stresses
[22]. When an OCXO is turned on, there can be a significant thermal-transient
effect. Figure 22 shows what happens to the frequency output of two OCXOs, each
containing an oven that reaches the equilibrium temperature in six minutes.
One oven contains an AT-cut, the other, an SC-cut crystal. Thermal gradients
in the AT-cut produce a large frequency undershoot that anneals out several
minutes after the oven reaches equilibrium. The SC-cut crystal, being "stress-compensated"
and thereby insensitive to such thermal-transient-induced stresses, reaches
the equilibrium frequency as soon as the oven stabilizes.

In addition to extending the warmup time of OCXOs, when crystals other than
SC-cuts are used, the thermal-transient effect makes it much more difficult
to adjust the temperature of OCXO ovens to the desired turnover points, and
the OCXO frequencies are much more sensitive to oven-temperature fluctuations
[22].

The testing and compensation accuracies of TCXOs are also adversely affected
by the thermal-transient effect. As the temperature is changed, the thermal-transient
effect distorts the static f vs. T characteristic, which leads to apparent hysteresis
[26]. The faster the temperature is changed, the larger is the contribution
of the thermal-transient effect to the f vs. T performance.